1. Technical Field
The present invention relates to a temperature sensor. More particularly, the present invention relates to a temperature sensor circuit for use in a semiconductor integrated circuit.
2. Description of the Related Art
Various semiconductor devices embodied with integrated circuit chips, such as CPUs, memories and gate arrays etc., are incorporated in various electrical products such as portable personal computers, Personal Digital Assistants (PDAs), servers or workstations. While these electrical products have a sleep mode for saving electrical power, most circuit components become a turn-off state. However, a Dynamic Random Access Memory (DRAM) as a volatile memory should refresh data of memory cells by itself in order to continuously preserve the data stored in the memory cells. The self-refresh operation requires self-refresh electrical power in the DRAM. Meanwhile, however, reducing the electrical power in a battery operated system requiring a lower electrical power is very important and critical.
One technique to reduce the electrical power needed for the self-refresh is to change the refresh cycle in conformity with the temperature. The data preservation time in a DRAM becomes longer at reduced temperatures. Thus, the temperature range is divided into several regions, and the frequency of the refresh clock is lowered relatively at a low temperature region, to thus reduce the consumption of electrical power. Herewith, in order to obtain an internal temperature of a DRAM, a built-in temperature sensor having low electrical power consumption is required.
FIG. 1 illustrates a circuit configuration of a conventional temperature sensor using a band-gap reference circuit. Referring to FIG. 1, a temperature sensor 100 comprises a current mirror type differential amplifier DA, a decrease resistance (R1) branch in which the current decreases with an increase in temperature, an increase resistance (R) branch in which the current increases with an increase in temperature, and a comparator OP1 for outputting as a comparison output signal OUT a comparison result between a reference temperature ORef and a sense temperature OT1. P-type MOS transistors MP1, MP2, MP3 have a size ratio of 1:1:1, and N-type MOS transistors MN1, MN2, MN3 also have a size ratio of 1:1:1, wherein the size indicates the product of the channel length L and the gate width W.
The temperature sensor shown in FIG. 1 operates as follows. A current of IO:Ir=1:1 flows by a current mirror operation of the PMOS transistors MP1,MP2 and the NMOS transistors MN1,MN2 equipped within the differential amplifier DA, and the voltages at the branches A, B have the same level.
A current equation for a turn-on section in a general junction diode becomes I=Is{e(VD/VT)−1}≈Is*e(VD/VT), wherein Is indicates a reverse saturation current, VD is a diode voltage, and VT is kT/q and indicates a thermal voltage.
Voltages appearing in the branches A, B are the same as each other, thus VA=VB=VD1=VD2+Ir*R. Also, IO=Is1*e(VD1/VT)→VD1=VT*ln(IO/Is1).
Further, since Ir=Is2*e(VD2/VT)→VD2=VT*ln(Ir/Is2)=VT*ln(IO/IS2)=VT*ln(IO/M*Is1), where M is a natural number indicating a size ratio of the diodes D2 and D1, i.e., M=(size of D2)/(size of D1).
Therefore, VD1=VD2+Ir*R becomes VT*ln(IO/Is1)=VT*ln(IO/M*Is1)+Ir*R.
That is, Ir=VT*ln(M)/R. Accordingly, a current proportional to the temperature flows at the branch A. In addition, when a similar amount of current flows in I1 and IO, a voltage VC of branch C is about equal to a value of VB, and therefore VC=VD1=VT*ln(IO/Is).
In general, the reverse saturation current Is increases greatly in comparison with VT, according to the increase of temperature, thus a diode voltage has a characteristic of a reduction based on a temperature. In other words, since VC is reduced by the temperature increase, Il is reduced by increasing temperature.
Hence, a resistance value of the resistor R1 of the decrease resistance (R1) branch is tuned so that a value of Ir and Il cross at a specific temperature T1 as shown in FIG. 2. The temperature sensor 100 of FIG. 1 functions as a temperature sensor designed to have a trip point at the specific temperature T1. FIG. 2 is a graph showing a temperature-to-current change that appears in the resistance branches by operation of the temperature sensor of FIG. 1. In FIG. 2, if the specific temperature T1 is, e.g., 45° C., an output signal OUT outputted from the comparator OP1 within the temperature sensor 100 is provided as a waveform OUT as shown in FIG. 3. FIG. 3 illustrates an output waveform of the comparator through an operation of the temperature sensor of FIG. 1.
In applying a general built-in temperature sensor shown in FIG. 1 to a semiconductor memory device such as a DRAM, a temperature tuning operation is performed on the temperature sensor. Accordingly, devices constituting the temperature sensor have a characteristic sensitive to a manufacturing process change so as to change the trip point. The temperature tuning process wherein the changed trip point is matched to a designed temperature point, is generally performed for every separate chip at a wafer level; and a process of detecting a shift temperature shifted by the manufacturing process change, and an operation of performing a temperature trimming through a cutting of device such as a fuse etc., are executed sequentially.
Herewith, the temperature sensor of FIG. 1 has only one decrease resistance branch, thus there is only a trip point for one specific temperature. Thus, refresh cycles controlled by a temperature provided before/after a specific temperature have a remarked difference. For example, if the specific temperature is 45° C., the refresh cycle is relatively lengthened from 1° C. to 44° C., but is relatively shortened at 46° C. that is a temperature greater than the trip point.
Thus, in order to mitigate the remarked difference of the refresh cycles at temperatures greater than, and less than, the specific temperature, a separate decrease resistance branch is connected in parallel with the decrease resistance branch of the temperature sensor in the prior art. For instance, in order that the temperature sensor has two trip points, another branch should be arranged in parallel with the branch C of FIG. 1, and the branch should be connected to a resistance. Hence, resistance branches corresponding to the necessary number of trip points are installed in order to have more than two trip points.
However, in a general control of the refresh cycle, the temperature sensor having two trip points is used, which is why there are problems such an increased time to perform a resistance trimming operation based on a process change, and an extension of the area occupied by the temperature sensor, in case the temperature sensor is provided with more than two branches.
That is, there was the difficulty to have numerous trip points in the conventional temperature sensor. In employing the conventional temperature sensor in the semiconductor memory device, a refresh cycle of the semiconductor memory is difficult to be appropriately controlled by a temperature change without a rapid change of a refresh cycle control, thus reliability of a semiconductor device may be dropped.
Hence, it would be desirable to provide an improved temperature sensor and temperature sensing circuit capable of having numerous trip points even without an extension in the number of decrease resistance branches in the temperature sensor.
Some example embodiments of the present invention provide a built-in temperature sensor and a temperature sensing circuit having the temperature sensor, which is adaptable to the interior of a semiconductor integrated circuit and has numerous trip points without extending the number decrease resistance branches in the temperature sensor.
Some example embodiments of the present invention provide an on-chip temperature sensing circuit for adequately controlling a refresh cycle in conformity with a temperature change in equipping the temperature sensing circuit with the semiconductor memory device.
Some example embodiments of the present invention provide a built-in temperature sensing circuit capable of substantially reducing a self-refresh electrical power consumption of a semiconductor memory device in conformity with a temperature change.
In one aspect of the invention, a temperature sensing circuit comprises sampling signal generator adapted to generate a reset signal and sampling signals; a temperature sensor, comprising, a decrease resistance branch in which current decreases in response to an increase in temperature, and a current mirror differential amplifier connected to the decrease resistance branch, wherein said temperature sensor is adapted to output temperature sensing data generated in response to the sampling signals; and a counting output part adapted to count and latch the temperature sensing data from the temperature sensor, and adapted to output counting data, said counting output part being reset by the reset signal of the sampling signal generator.
Beneficially, the differential amplifier includes an increase resistance terminal and a decrease resistance terminal, and the temperature sensor further comprises an increase resistance branch, which is connected between the increase resistance terminal of the differential amplifier and a lower supply voltage, in which current increases in response to an increase in temperature; a first resistance string having a plurality of resistances connected in series with the decrease resistance branch, between the decrease resistance terminal and the lower supply voltage; a short-circuit switching part adapted to selectively short-out one or more of the plurality of resistances in response to the sampling signals; and a comparator adapted to compare a reference temperature output appearing at the increase resistance with a sensing temperature output appearing at the decrease resistance terminal, and outputting the comparison result as the temperature sensing data.
Beneficially, the sampling signal generator comprises a plurality of unit sampling signal generating parts connected in cascade, each said unit sampling signal generating part being adapted to apply a NOR operation to an applied input signal and an inverted and delayed signal of the input signal, and outputting a result of the NOR operation as a corresponding one of the sampling signals, wherein a first unit sampling signal generating part outputs a first sampling signal, a second unit sampling signal generating part outputs a second sampling signal, the last unit sampling signal generating part outputs the reset signal, and a unit sampling signal generating part immediately preceding the last unit sampling signal generating part outputs a pass gate control signal.
Also beneficially, the counting output part comprises a plurality of flipflops connected in cascade, each flip-flop having an input terminal, an output terminal, and a clock terminal, wherein the input terminal of each flipflop is connected to a fixed voltage level, wherein the temperature sensing data is received at the clock terminal of a first-stage flipflop, and wherein an output signal appearing at the output terminal of each flip-flop is connected with the clock terminal of a subsequent flipflop; a pass gate adapted to transmit the output signals of the plurality of flipflops in response to the pass gate control signal; and a latch adapted to latch the outputs of the plurality of flipflops transmitted by the pass gate.
Beneficially, the temperature sensing circuit also includes an oscillator outputting an oscillation signal having an oscillating period controlled in response to the counting data output by the counting output part.
In one embodiment, the oscillator comprises an inverter chain having an odd number of inverters; a capacitor connected between an output terminal of the inverter chain and a lower supply voltage; a plurality of resistances cascade-connected between inverters of the inverter chain; and switching transistors connected in parallel across resistances, said switching transistors being adapted to selectively short-out the resistances in response to the counting data.
Also beneficially, the temperature sensing circuit also includes a refresh counter refresh counter comprising a plurality of flipflops connected in cascade, each said flipflop having an input terminal, an output terminal, and a clock terminal, wherein the input terminal of each flipflop is connected to a fixed voltage level, wherein the oscillation signal is received at the clock terminal of a first-stage flipflop, and wherein an output signal appearing at the output terminal of each flip-flop is connected with the clock terminal of a subsequent flip-flop.
Also beneficially, the temperature sensing circuit also includes a resistance value trimming part connected to the first resistance string and adapted to individually vary resistance values for respective resistances of the first resistance string part.
Beneficially, the temperature sensor can further include a second resistance string having a plurality of resistances connected in series with the decrease resistance branch, between the decrease resistance terminal and the lower supply voltage; and a short-circuit release switching part adapted to selectively disconnect a short across one or more of the plurality of resistances of the second resistance string in response to the sampling signals.
In another aspect of the invention, a temperature sensor, comprises a decrease resistance branch having a first current that decreases in response to an increase in temperature, the decrease resistance branch comprising, a plurality of resistors connected in series, and means for selectively shorting-out one or more of the plurality of resistors to adjust a total series resistance in the decrease resistance branch; an increase resistance branch having a second current that increases in response to an increase in temperature; a current mirror differential amplifier having an increase resistance terminal connected to the increase resistance branch, a decrease resistance terminal connected to the decrease resistance branch, and adapted to output a first voltage representing a reference temperature corresponding to the first current and a second voltage representing a sense temperature corresponding to the second current; and a comparator adapted to compare the first voltage and the second voltage and to output an output signal having a first logic level when the first voltage is greater than the second voltage, and a second logic level when the first voltage is less than the second voltage.
Beneficially, the means for selectively shorting-out one or more of the plurality of resistors comprises a plurality of switches connected in parallel across resistances, said switches being adapted to selectively short-out the resistances in response to control signals.
Beneficially, the decrease resistance branch further comprises a resistance value trimming part connected across the plurality of resistances and adapted to individually vary resistance values for respective resistances.
Beneficially, the resistance value trimming part comprises a plurality of fuses.
Beneficially, the comparator has an enable input and outputs the output signal at a fixed logic level in response to the enable signal being inactive, regardless of the first and second voltages
Accordingly, numerous trip points can be provided by a temperature change without extending the number of decrease resistance branches, that is, a required fine control can be performed in conformity with the temperature change.